The present invention relates to an optical recording medium applied to record and erase information by use of a recording layer changing its crystallographic state.
Highlight is recently focused on optical recording media which can record information at a high density and erase the recorded information for rewriting. One typical rewritable optical recording medium is of phase change type wherein laser light is directed to the recording layer to change its crystallographic structure whereupon a concomitant change of reflectivity is detected. Optical recording media of the phase change type are of great interest because either recording or erasing may be chosen upon irradiation of a light beam simply by changing the intensity thereof, apparent overwrite recording is then possible with the use of a single light beam, and the drive unit has a simple optical system as compared with that of magnetooptical recording media.
Ge-Te materials have mostly been used for the phase change type of optical recording media because there is a large reflectivity difference between their crystalline and amorphous states and they are relatively high in terms of the stability of their amorphous state. More recently, it has been proposed to apply compounds called chalcopyrites.
Chalcopyrite type compounds have been investigated as compound semiconductor materials and have had application for solar batteries, etc. Referring now to the chemical periodic table, they have a composition represented by Ib-IIIb-VIb.sub.2 or IIb-IVv-Vb.sub.2 and have a structure that two diamond structures are stacked up together. The chalcopyrite type compounds can easily be structurally identified by X-ray structural analysis with the basic characteristics described in periodical publications "Physics", Vol. 8, No. 8, page 441 (1987) and "Electrochemistry", Vol. 56, No. 4, page 228 (1988) for instance.
Of these chalcopyrite type compounds, AgInTe.sub.2 in particular is known to be capable of being used as a recording layer material for an optical recording medium used at a linear velocity of about 7 m/s, if it is diluted with Sb or Bi. See Japanese Patent Application Kokai(JP-A) No. 240590/1991, 99884/1991, 82593/1991 and 73384/1991 among which JP-A 240590/1991 in particular comes up with an information recording medium having a recording layer composed predominantly of (AgInTe.sub.2).sub.1-a M.sub.a where M is Sb and/or Bi and 0.30.ltoreq.a.ltoreq.0.92 and comprising a mixture of AgInTe.sub.2 and M phases. The alleged advantages include improvements in sensitivity to laser writing, erasability, recording/erasing repeatability and erasing rates.
In recent years, highlight is also focused on optical recording disks which are recordable and reproducible at the same linear velocity (of about 1.2 m/s to about 1.4 m/s) as compact disks (CDs) for the reason that they can be used in combination with CDs and their drive units only by addition or modification of optical systems. For such optical recording disks, for instance, optical recording or magneto-optical recording disks of the write-once type in which organic dyes are used for recording layers have been developed. However, the above optical recording disks of the phase change type are now considered well suitable for such application because they are one beam overwritable and the optical system of the driving units used therewith is simple in construction.
JP-A 240590/1991 mentioned above describes in its example that the disk is rotated at a linear velocity of 7 m/s for recording. However, it has now been found that with the disk rotated at the same linear velocity as CDs, the obtained C/N is much lower than that obtained at a linear velocity of 7 m/s with a lowering of repetitive recording properties or characteristics. When an optical recording disk of the phase change type is used at a slow linear velocity such as one used for CDs, the heat of a laser beam has an influence on a region wider than the area irradiated with the laser beam. In the case of an optical recording medium of the phase change type prepared by use of a recording material based on (AgInTe.sub.2)M, the area heated by a laser beam is rapidly cooled into a state so amorphous or microcrystalline that signals can be recorded thereon. In a recording region with long time 11T signals recorded thereon, the irradiation finish area remains slightly heated under the influence of an irradiated area adjacent thereto, so that it can remain slowly cooled, if the linear velocity is slow. Neither good C/N nor repetitive recording properties are consequently obtainable.
To provide a solution to such problems, it is required to use a material having a low crystal transition rate (at which an amorphous or microcrystalline phase grows into a coarse crystal phase) for a recording layer, thereby making it amorphous or microcrystalline even when the cooling rate is low.
In view of such considerations, we have come up with a recording layer having a controlled composition containing Ag, In, Te and Sb to which there is additionally added at least one element M selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Mn, W and Mo, as described in "Jap. J. Appl. Phys.", 32(1993), 1980. The crystal transition rate of the recording layer is so reduced that the C/N and repetitive recording properties can be improved. This recording layer contains elements forming the above chalcopytite compound, while the unrecorded region, for the most part, is made up of AgSbTe.sub.2 (or a composition close thereto), InTe and Sb phases.
For a phase change type of optical recording medium, it is required that to increase signal strength, there be an increased difference (the degree of modulation) between the reflectivity of the crystalline state and the reflectivity of the amorphous or microcrystalline state. The as-formed recording layer by sputtering, because of being in an amorphous state, is first initialized. Initialization is operation for making the recording layer crystalline by heating and cooling. When signals are recorded on the as-initialized recording layer, however, the reflectivity of the area irradiated with recording light becomes higher than that of the as-formed area. This offers a problem that no theoretically possible degree of modulation can be obtained.
The reason that the reflectivity of the amorphous or microcrsytalline area formed by irradiation with recording light becomes higher than that of the as-formed area is that recrystallization, if not large, occurs when the recording layer melted by recording light is cooled. Such recrystallization becomes noticeable especially when recording occurs at a low linear velocity such as one encountered in the case of CDs. The reason is that the low linear velocity makes the post-melting cooling rate low. Such a degree-of-modulation reduction becomes considerable upon repetition of recording; that is, some limitation is imposed on the number of repetitive recording cycles. The reduction in the degree of modulation may possibly be limited by regulating the composition of the recording layer to decrease the crystal transition rate. However, the reduction in the crystal transition rate, in turn, makes it difficult to erase the recorded signals. Even when relying upon the recording layer in the above "Jap. Appl. Phys.", 32 (1993), 1980, difficulty is thus involved in increasing both the degree of modulation and the erasing rate to sufficient levels.